If the laws of nature are fine-tuned for life, can we infer other universes with different laws? How could we even test such a theory without empirical access to those distant places? Can we believe in the multiverse of the Everett interpretation of quantum theory or in the reality of other possible worlds, as advocated by philosopher David Lewis? At the intersection of physics and philosophy of science, this book outlines the philosophical challenge to theoretical physics in a measured, well-grounded manner. The origin of multiverse theories are explored within the context of the fine-tuning problem and a systematic comparison between the various different multiverse models are included. Cosmologists, high energy physicists, and philosophers including graduate students and researchers will find a systematic exploration of such questions in this important book.

‘In recent years multiverse theories have attracted a great deal of attention among physicists, promising simple resolutions to fundamental long-standing problems. Philosopher of science Simon Friederich provides here a valuable, careful examination of these claims and their relation to testable science. The problems posed by the advent of such theories that arguably cannot be shown to be wrong seem to be here to stay. Friederich's comprehensive and even-handed account of all sides of the question of where we are now with this new sort of science has an important role to play.’

Peter Woit - Columbia University

‘Friederich offers an excellent even-handed philosophical discussion of multiverse theories. By providing the first single-authored philosophical book on the topic, he moves multiverse issues towards the center of contemporary philosophy of physics and demonstrates that physical thinking about the multiverse may profit from philosophical considerations. The book will be of profound interest both for philosophers and physicists.’

Richard Dawid - Stockholm University

‘For those who are sceptical or agnostic about a multiverse, this book brings many interesting questions to light, and it certainly issues challenges to the enthusiast. Physicist readers may find the philosophical approach … demanding at times, but the writing is very clear on the whole, with a pleasant style, and Friederich maintains a balanced position among the many different points of view that he addresses … I would certainly recommend this ground-breaking book for any reader who is interested in cosmology.’

Peter J. Bussey Source: Contemporary Physics

‘The book is well written. I recommend it highly.’

Phillip Helbig Source: The Observatory magazine

'Friederich (Univ. of Groningen) explores the latter concept in this book, paying particular attention to the meaning of probability as it applies to the possible values of the fundamental constants, the concept of fine-tuning, and the testability of multiverse theories.'

A. Spero Source: Association of American Publishers

‘This book is one of the best introductions to the significance of this topic for theology today. The exhaustive bibliography of science, philosophy,history, epistemology, and mathematics at the end of the volume is, by itself, worth the price of this book.'

Nick Overduin Source: Calvin Theological Journal

‘… Friederich makes a significant up-to-date contribution to the ongoing de-bate surrounding the possible existence of a multiverse, a gigantic, usually infinite, collection of universes. He professes to do so as an unbiased philosopher with no axe to grind, and his treatment does indeed seem a very fair assessment of different sides of the debate.’

Rodney Holder Source: Science and Christian Belief